This invention relates to an internal combustion engine, and more particularly to a multicylinder two-stroke cycle engine in which a separate carburetor or other air-fuel supply means is provided for each cylinder.
In a multicylinder two-stroke cycle engine, which is typically employed in the power head of an outboard marine motor, it is known to provide one carburetor for each cylinder for maximum power output. With such an arrangement, air-fuel mixture is drawn into the cylinder crankcase compartment from the carburetor during the upward compression stroke of the piston. The air-fuel mixture typically passes through a manifold disposed between the carburetor and the crankcase. A reed valve or other such one-way valve is installed at the inlet to the cylinder crankcase compartment to provide one-way flow of air-fuel mixture thereinto. During the power stroke of the piston, the reed valve closes to prevent back flow of air-fuel mixture into the manifold. This cycle constantly repeats, resulting in intermittent drawing of air-fuel mixture from the carburetor.
As a result of the above-described operation, the carburetor associated with each cylinder is subjected to an on-again off-again suction resulting from the vacuum pulse of the piston during its compression stroke. This is in direct contrast to the situation in which a multicylinder engine is provided with a single carburetor. With such an arrangement, the carburetor is constantly subjected to a vacuum pulse from one cylinder or another, resulting in a substantially constant supply of air-fuel mixture therefrom.
It has been found that, in a two-stroke cycle engine in which a single carburetor provides air-fuel mixture to a single cylinder, the on-again off-again suction of air-fuel mixture from the carburetor provides certain undesirable operating characteristics. Illustratively, when the reed valve is closed and flow into the cylinder prevented, the air-fuel mixture can "bounce" off the closed reed valve due to its inertial mass. This results in back flow of the air-fuel mixture through the carburetor and spit back of the air-fuel mixture through the carburetor inlet. The inertia of the back flowing air-fuel mixture must then be overcome by the vacuum pulse created by the piston in its upward stroke in order to draw the air-fuel mixture through the reed valve and into the cylinder crankcase compartment. This results in a decrease in engine performance.
One solution to this problem has been to provide an unrestricted path of communication between the intake lines or manifolds downstream of the separate carburetors. This arrangement allows the vacuum pulse from one cylinder to suck air-fuel mixture not only from the carburetor to which it is directly connected, but also from the line interconnecting the remaining carburetors in the system. The air-fuel mixture in this line reciprocates back and forth depending on the source of the vacuum pulse at a particular point in time. Accordingly, each carburetor is subjected not only to the vacuum pulse from its associated cylinder, but also to a small degree to the vacuum pulses from the other cylinders.
It is an object of the present invention to provide a more satisfactory solution to the above-described problems than that provided by the prior art. More particularly, it is an object of the invention to specifically direct the vacuum pulse from one cylinder in a multicylinder system not only to its primary carburetor, but also to another carburetor in the system to relieve back pressure in the intake line connected to such secondary carburetor. This system reduces back flow through the secondary carburetor by subjecting it to a specifically directed vacuum pulse at a time when the secondary cylinder would not otherwise be subjected to a vacuum pulse from its cylinder.
In accordance with the invention, vacuum pulse balancing means is provided for a multicylinder two-stroke cycle engine in which each cylinder is provided with a separate air-fuel mixture supply means and intake means for supplying air-fuel mixture from the air-fuel supply means to its associated cylinder. The vacuum pulse balancing means comprises a plurality of passages disposed downstream of each air-fuel mixture supply means. The passages place each air-fuel mixture supply means in communication with a cylinder other than the cylinder with which the air-fuel mixture supply means is primarily associated. One-way valve means is provided within each passage for transferring a vacuum pulse from one intake line to another in a proper sequence, thereby increasing the amount of time to which the secondary carburetor is subject to a vacuum pulse. Back flow of air-fuel mixture in the intake line associated with the secondary carburetor is thereby relieved.